Canisters for adsorptive control of fuel vapors are well known in the automotive arts. A typical automotive emissions control canister comprises a housing having an inlet and outlet and a chamber for holding a charge of activated carbon. The inlet is connected to the headspace in the vehicle's fuel tank and the outlet is vented to atmosphere. In addition, the canister has a purge tube located on the inlet end of the canister which is connected to a vacuum source on the engine. When the fuel vapor volume expands thermally in the tank, the displaced fuel vapors are adsorbed by the activated carbon bed. When the engine is operated, the engine vacuum is applied to the purge tube and air is drawn through the carbon bed, desorbing the adsorbed vapors and carrying the vapors into the engine's intake manifold.
At the present time, emissions control canisters are not used with inboard marine applications. When boats having fixed fuel tanks are subjected to diurnal temperature changes, the displaced vapors are passed undesirably into the atmosphere. It is believed that US Federal law will require for model year 2011 that vessels having fixed on-board fuel tanks have evaporative emission control.
Prior art automotive-type canisters are not readily adaptable to use in boats. Such canisters are bulky and typically have a U-shaped vapor path with inlets and outlets formed at the top. On a marine vessel, the tank vent outlet typically is located high on the vessel topsides just below the rail, to avoid taking in water during vessel use. Thus, a suitable marine-use canister would have a vapor inlet at the bottom, for direct vapor flow from the fuel tank, and an outlet at the top, for connection to the vapor through-hull fitting. Of course, canisters may be mounted horizontal as well.
Further, most small pleasure boats having onboard fuel tanks are formed by assembling a molded inner hull, containing the decking and vessel superstructure, to a molded outer hull containing the engine mounts and through-hulls. Typically, a dead space is formed between the inner hull and outer hull above the waterline. Such space is available and strategically ideal for mounting a marine emissions adsorption canister; in fact, prior art vent lines typically pass through this space. However, such a canister must have a relatively small diameter, preferably less than about 4 inches, to fit easily into this space.
Another disadvantage of trying to adapt an automotive canister to marine use is that such canisters are manufactured in expensive, complicated molds which are justified only because many identical canisters are required for an entire line of automobiles, whereas the total annual volume of boats is only about 550,000 manufactured by over 1200 boat builders producing many thousands of lines of boats, each of which has different dimensions for the between-hull space. Thus, prior art automotive canisters, which may number in design no more than a few dozen or so in any model year, are not readily adaptable to fit into the huge number of different spaces presented by the boat industry.
An additional disadvantage is that the carbon beds in prior art automotive canisters are vulnerable to contamination by water in a marine environment, reducing the effectiveness of adsorption.
A still further disadvantage is that prior art automotive canisters are significantly more complicated than will be required, at least initially, for marine fuel tanks. A marine emissions canister may be inserted into the prior art vapor exhaust flow path, and is regenerated passively by the inhalation of air into the fuel tank as the fuel tank cools down during the diurnal. Thus, a marine canister can be significantly simpler and less expensive than an automotive canister.
A prior art linear canister having an inlet at one end and an outlet at the other is disclosed in U.S. Pat. No. 6,537,355, the relevant disclosure of which is incorporated by reference herein. A carbon monolith is disposed within a two-part cylindrical shell and is insulated and suspended therein by resilient annular spacers which also prevent bypassing of fuel vapors.
The disclosed canister is intended for automotive uses and therefore suffers from most of the above-recited shortcomings although it is linear and relatively slim. However, the housing is formed by injection molding in expensive molds to provide integral features for joining the shell halves together. Thus, the overall length and capacity of the canister is not easily or economically changed to accommodate different between-hull spaces. Further, the carbon monolith, although extremely efficient in scavenging fuel vapors, is both expensive and delicate; hence the need for resilient, insulative spacers. Initial marine requirements can be met by significantly simpler, less expensive forms of activated carbon.
It is a principal object of the present invention to provide a simple emissions control canister meeting anticipated marine requirements.
It is a further object of the invention to simply, reliably, and inexpensively adsorb fuel vapor emissions from a fixed fuel tank on a marine vessel.